There are a ton of possibilities here to configure different tones on your guitar if you care to experiment.
In this instructable, I'll show how I installed a switch to make both of the humbuckers on my guitar operate as single coils at the same time, but I'll also describe some other options in detail. For example, It is possible to create a single pickup that operates as both a single coil AND humbucker with a control to blend how much of each type of sound is routed through the guitar's controls to the amp. Using a control scheme like this on all of the available pickups would create many tonal variations that can be easily tweaked during a performance. Another possibility, would be to use the pickup as "mostly" single coil to get that single coil tone that is often desired, but use the blend control to dial out some of the characteristic hum at higher gain levels. This would take advantage of the humbuckers hum cancelling properties as much or as little as required for the current volume/gain settings.
In it's simplest form this mod is very easy. If all you want is to turn your humbuckers into single coils you can pull that off in about 20 minutes with nothing more than a small piece of wire! Adding a switch will be more challenging mostly because you need to find a place to put it. You can get a push/pull type potentiometer/switch to replace your volume POT, or you may have to cut a hole in the pick guard to mount a toggle switch. If you intend to blend the two coils you will need a push/pull pot installed in addition to whatever knobs you already have, so make sure you have the necessary space for whatever new components you'll be adding before you blow the money on parts... In my case, there wasn't much room and I didn't have a suitable push/pull POT in my collection, so I added a discrete sliding toggle switch between my existing volume/tone knobs. I had to cut a hole to fit the switch, but I wasn't modding a vintage '56 les paul goldtop or anything! I got this guitar at my local pawn shop for $60! So, take that into consideration when choosing to perform ANY modification. The push/pull volume POT is the way to go to keep this mod reversible if you have an expensive or vintage instrument and still want to make changes to it.
Step 1: Functional description
Since the 2 coils of the humbucker are wired in series, that means that one side of each coil is connected together, the other wire on coil #1 goes to ground, and the other wire on coil #2 is the output signal. So, if you connect a wire to the place where the 2 coils connect together and then connect that new wire to ground... coil #1 is grounded on both sides effectively eliminating it from the circuit leaving coil #2 to operate by itself! Have a look at the schematic pics... Simple right?
To go back and forth between single coil and humbucker operation we need to simply add a switch in that new wire!
Back to tonal characteristics for a minute. Obviously, since we took half of the magnets and half of the coil windings out of the circuit we now have half of the output (but not half volume because your ear's response to loudness logarithmic rather than linear, but that's another matter.) Notice that a coil of wire is an inductor and we took half of the inductance out too! The instrument cables we all use have inherent capacitance (about 15-30 pF per foot or so) that creates a tuned circuit with your inductive pickups! This tuned circuit varies with different cables and pickups and can produce a slight boost in the mids and roll off the highs depending on cable length and pickup inductance... The options described below could allow you to vary this tuning to change the frequency response of you rig.
To start really tweaking we could go a step further and add resistor to the wire. The greater the value of the resistance the more of Coil #1's signal will go through coil #2 and the more influence coil #1 will have to the overall circuit. With very low resistance here, coil #1 will have an easier path to ground, so it will have much less influence.
Want more? Why not make the resistor variable? Now coil #1 can influence the circuit as much or as little as we want!
Refer to the schematic pics for these options because I didn't do this on my guitar seen in the pics, and most of this is difficult to describe in writing. (The schematics do not show any volume/tone controls or pickup switching because every instrument will be different). I didn't have the desire or the space to add this much control, but I do want you to understand what is possible. It would take virtually no space to add a fixed resistor to tune the amount of coil #1 in the circuit, but I have not experimented with values for this resistor. If you wanted to try, I think the best way would be to temporarily wire a 1M or a 500K or so variable resistor and play with it until you find a setting you like. Then use a multimeter to measure the value and replace it with the nearest standard value resistor to complete the permanent installation.
If you try some of these options, I hope to see a report in the comments section! It would be very cool to hear about the results people are getting with different configurations.
Step 2: Getting started
Step 3: Working with the pickup
Often, the pickup will be wrapped in some tape that can be carefully removed to expose the wires. Sometimes, the pickup is sealed in such a way that you cannot get to the wires without damaging it. If this is the case for you, I recommend that you put it back together and save some $$$ for new pickups, or just live with the buckers! If you're able to remove the tape, or other covering, and get to the delicate wires then you can continue!
My pickup had 4 wires inside (perfect situation for this mod). There is a black and a white for coil 1 and a black and white for coil 2. The 2 white ones were soldered together completing the series connection between the coils, and the 2 blacks were connected to ground and the pickup selector switch. The one going to the pickup selector switch is the one carrying the signal to the rest of the electronics and on to the amp. The colors of the wires do not matter, and will likely be different for you depending on the pickup manufacturer. The thing to look for is the connection between the two coils. If you pickup only has 2 wires, you may have to look for a small wire joining the coils that is not very accessable. If it is long enough, and you're brave enough, you can cut it to create a new point to solder to. I've added a pic of color codes to aid you in case you might need it.
I should point out here that the windings inside the pickup are extremely delicate! do not pull on the wire at all! If one of your wires gets pulled out, it's unlikely that you'll be able to find the tiny winding wire and solder it back without damaging it beyond repair! So, be careful. As long as you apply roughly ZERO tension to the wires you should be ok.
Now, we can simply solder a small piece of wire to the same point where the two coils are connected together. If your pickup is like mine, that just involves soldering another wire onto the same joint where the 2 white wires meet. Whatever the case, you need to get a new wire soldered at the connection point between the two coils.
After that is complete, you can wrap the tape back around the pickup. Or, put the plastic shell back on, or whatever. After this one new wire, we're done inside the pickup!
At this point, if you don't want a switch to toggle between single coil/humbucker operation then all you have left to do is solder the other end of the new wire to a nearby ground (I used the case of the volume pot where there was already an existing ground connection) and put it all back together. The next step goes into more detail about adding the switch for 2 pickups.
Step 4: Adding switch control
If your not lucky enough to be replacing an existing POT with a push/pull POT, then you'll have to find a place to install your new switch or new POT. You'll have to carefully drill the hole and put it in it's place. I used a little slide toggle that needed a rectangular hole, so I cut it out with a dremel.
If you're just adding a switch like I did, then all that's left to do is connect the new wire from the pickup to one side of the switch and a wire from the other side of the switch to ground. My switch is a DPDT type so that I can connect both of my pickups this way. I had considered 2 switches so that I could change them independently, but I was lacking the space inside the cavity of my guitar.
That's it! I encourage you to explore some of the other options in the schematic drawings though. There might even be a great idea that i missed!
Step 5: Conclusions
I have an FX box that has a humbucker-to-single coil emulator in it that doesn't make nearly the amount of tonal change that doing this simple mod does. It works best clean to slightly overdriven through a tube amp. At high gain settings the hum is a little too much, but that's why you installed the switch! right?